The production of shaped polyurethane foam articles having a compact surface by foaming in molds is known (see, for example German Auslegeschrift No. 1,196,864). A reactive and foamable mixture based on compounds containing isocyanate-reactive hydrogen atoms and polyisocyanates is introduced into a mold. In conventional processes, water and/or fluorochlorinated hydrocarbons are used as blowing agents. Catalysts of the type known in the production of polyurethane foams are also generally used.
By suitably selecting the starting components and, in particular, by suitably selecting the molecular weight and the functionality of the various components, it is possible to produce foams ranging from elastic to rigid. In this process, the compact outer skin is obtained by introducing into the mold a larger quantity of foamable mixture than is required to fill the mold by free foaming. The inner wall of the mold generally cools the reaction mixture over its inner surface and causes condensation of the preferred organic blowing agents. The blowing reaction comes to a stop at the inner wall of the mold and a compact outer skin is formed.
The organic blowing agents used for this process are halogenated hydrocarbons since they have a low boiling point and do not form explosive mixtures with air. Pentane, for example, is difficult to use as a blowing agent because elaborate safety precautions would have to be taken due to the low explosion limit.
Proven blowing agents for polyurethane foams include fluorotrichloromethane and/or methylene chloride. Efforts are being made to replace these blowing agents by blowing agents free from fluorinated hydrocarbons for reasons of pollution control, however. Accordingly, it has become desirable to develop alternative blowing agents for the production of cellular and porous plastics articles.
As already mentioned, water is often used as a blowing agent in the production of polyurethanes. Although it is possible by using water to produce polyurethane free-rise foams of outstanding quality, water is not suitable for the production of integral skin foams because both the surface and the integral structure of the foam are decidedly inferior when compared with integral skin moldings foamed with fluorinated hydrocarbons. The carbon dioxide given off when water is used as the blowing agent does not condense on the cool inner wall of the mold. Because of this, there is no formation of the desired compact outer skin. Further, in the reaction between isocyanate and water, amino groups are formed from the isocyanate groups and immediately react with the excess isocyanate groups, increasing the viscosity of the reaction mixture.
Water is generally added to the reaction mixture as an individual component immediately before foaming. When water is added to the polyol component which normally already contains the tin compounds (such as dibutyl tin (IV) dilaurate) essential as foaming catalysts, the tin compounds can undergo at least partial hydrolysis, resulting in an uncontrolled reduction in the activity of the completed polyol component.
Other alternative blowing agents are compounds which decompose at temperatures above room temperature and, in doing so, give off a blowing gas. Examples of these compounds include azodicarbonamide, azo-bisisobutyronitrile or diphenylene oxide disulfohydrazide which give off nitrogen under the effect of heat. Compounds which give off carbon dioxide include pyrocarbonic acid esters and anhydrides (German Offenlegungsschrift No. 2,524,834/U.S. Pat. No. 4,070,310) and benzo-oxazines (German Auslegeschrift No. 2,218,328).
These compounds must have a relatively low decomposition temperature which should generally be below 100.degree. C. since the blowing agents must be active at the beginning of the urethanization reaction when the heat of reaction is still at a relatively low level. However, compounds having such a low decomposition temperature are naturally sensitive during storage and require careful handling which in many cases cannot be guaranteed during the processing of polyurethane foams by fabricators. In addition, these compounds may undergo uncontrolled decomposition during storage.
It is also generally known that carboxyl groups react with isocyanates to give off CO.sub.2 and thus are capable of contributing to the blowing reaction in polyurethane plastics (O. Bayer, Angewandte Chemie, 59, 267 (1947)). However, this reaction is extremely involved (particularly for aromatic isocyanates of the type used commercially), leads to discoloration in polyurethane, and is generally not adequate for blowing reactions on a commercial scale.
In order to ensure effective synthesis of the polyurethane in the polymer-forming reaction, it is therefore normally necessary to use urethane compounds which contain only terminal OH-groups. In the case of polyester polyols, they may also contain a few terminal carboxyl groups (acid numbers in the usual range from about 0.1 to 2). However, these small numbers of terminal carboxyl groups are of very minor significance to the blowing reaction. Polyester polyols of this type are preferably used in the production of non-cellular polyurethane elastomers. If polyesters containing predominant amounts of terminal carboxyl groups are used, the generation of CO.sub.2 is still inadequate. Additionally, synthesis of the polymer follows a different course and gives product properties differing from polyurethanes.